Not Applicable
Not Applicable
The present invention is directed generally to communication networks and systems. More particularly, the invention relates to optical WDM systems and optical components, such as add/drop devices, employing multiple wavelength detection and processing.
Optical communication systems transport information by generating optical signals corresponding to the information and transmitting the optical signals through optical transmission fiber. Information in various formats, such audio, video, data, or any other formats can be optical transported through many different networks, such as local and long distance telephone, cable television, LAN, WAN, and MAN systems, as well as other communication networks.
Optical systems can be operated over a broad range of frequencies/wavelengths, each of which is suitable for high speed data transmission and is generally unaffected by conditions external to the fiber, such as electrical interference. Also, information can be carried using multiple optical wavelengths that are combined using wavelength division multiplexing (xe2x80x9cWDMxe2x80x9d) techniques into one optical signal and transmitted through the optical systems. As such, optical fiber transmission systems have the potential to provide significantly higher transmission capacity at a substantially lower cost than electrical transmission systems.
However, optical transmission systems are not free from various forms of degradation that limit the performance of the systems. For example, optical fiber is not a perfect transmitter of electromagnetic radiation in the optical spectrum. Thus, the intensity of an optical signal is attenuated as it travels through the fiber, due to diffraction from fiber material imperfections and other degradation mechanisms. Furthermore, optical noise from signal attenuation and optical components in the system will accumulate and propagate in the fiber, and chromatic dispersion and nonlinear signal interactions will degrade the quality of the signal. In addition, optical systems are generally not operated in the identical manner or using a common set of wavelengths that would facilitate optical transmission across interfaces between optical systems.
It is therefore necessary to regenerate optical signals being transmitted through the optical system to overcome the three primary limitations on optical transport, namely: 1) optical signal attenuation, 2) optical signal degradation, and 3) optical system interconnectivity. The regeneration of optical signals can be performed either optically or electrically.
In early optical communication systems, there was no commercially viable method of optically amplifying or regenerating optical signal to compensate for signal attenuation. Therefore, optical signals had to be electrically regenerated by converting the optical signal into an electrical signal, while the signal intensity was sufficiently high and the optical noise sufficiently low that the signal could be detected. The electrical signal would then be amplified, further processed, if desired, converted back to an optical signal and transmitted through the next span of fiber. Electrical regenerators of this type are commonly referred to as 3R (reshape, retime, regenerate) repeaters.
The general configuration of a 3R repeater includes a number of optical receivers electrically connected to a corresponding number of optical transmitters. The number of receiver/transmitter pairs corresponding to the number of wavelengths used in the optical system.
The optical receivers generally include optical to electrical converters, such as a photodiodes, configured to receive an information carrying wavelength from the optical fiber and provide a corresponding information carrying electrical signal. Electrical amplification, and other processing, such as retiming, reshaping, electrical add/drop multiplexing and demultiplexing, etc. are performed on the electrical signal as necessary.
The optical transmitter converts the regenerated electrical signal into an optical signal and transmits the optical signal into the next optical fiber span. The electrical to optical conversion at the transmitter is typically performed by either directly or externally modulating an optical signal laser with the regenerated electrical signal. Electrical regenerators are well known in the art, for example, see U.S. Pat. Nos. 5,504,609; 5,267,073; 4,663,596; 4,549,314; 4,313,224; 4,307,469; 4,295,225; 4,234,970; 4,075,474; 4,019,048; 4,002,896; and, 3,943,358.
Electrical regeneration of the optical signals generally as required every 30-40 km to compensate for signal attenuation and signal degradation. Therefore, in order to span distances in excess of 30-40 km between an origin and a destination, it was necessary to serially connect fiber spans and electrical regenerators. The cost of frequent regeneration limited most early optical systems to operation using a single wavelength.
While frequent electrical regeneration greatly increases the overall transmission cost, the use of electrical regenerators does allow for each optical fiber span to be operated optically independent of the other spans. Accordingly, the wavelength of the optical signal used in each span could be varied depending upon the transmission characteristics of each span.
For example, many early and current optical systems operate in the 1300 nm range to minimize the degradation of the optical signal due to chromatic dispersion that occurs in the optical fibers. While other optical systems are operated in the 1550 nm range, which minimizes the optical signal attenuation in the fiber.
Electrical regeneration of optical signals to interface with another optical systems is generally performed as part of the electrical regeneration performed to compensate for signal attenuation and/or degradation. In this manner, the optical links can be established to minimize the number of regeneration points required between the signal origin and destination.
The advent of optical amplifiers, semiconductor and Erbium doped optical amplifiers, or EDFAs, provided a means to optically regenerate the optical signal to compensate for fiber attenuation of the optical signal. It was therefore possible to replace electrical regenerators used to compensate for fiber attenuation in the optical system, as discussed in U.S. Pat. No. 4,947,134 issued to Olsson et al.
A difficulty with optical amplified systems is that optical noise accumulates in the fiber and chromatic dispersion and non-linear interactions degrade the signal quality as the signal propagates in the fiber. The increasing noise levels and signal degradation produce a corresponding increase in the bit error rate of the system. As such, while the optical amplifiers have greatly reduced the need for electrical regeneration, optical systems still require electrical regeneration to eliminate accumulated optical noise and maintain signal quality, as well as interface with other optical systems.
A further advantage of optical amplifiers over electrical regenerators is that a plurality of wavelengths can be optically amplified at one time with only minimal additional expense. The primary additional expense is the cost of providing optical energy, i.e., pump power, to the optical amplifier, which increases with the number of wavelengths and/or the gain of the amplifier.
The ability of optical amplifiers to optically regenerate optical signals over a range of wavelengths has dramatically decreased the number of electrical regenerators required in optical systems. The reduction in electrical regeneration requirement has dramatically increased the commercial viability of WDM systems as a cost effective means of adding capacity to optical systems.
However, the number of wavelengths used in a WDM system is limited to specific wavelength range in which the optical amplifiers can amplify optical signals. The number of channels is also limited by the spacing of the wavelength channels in the WDM system.
The channel spacing in optical systems is limited by a number of factors, one of which is the modulation technique used in the optical transmitter. For example, direct modulation of the laser is the most cost effective technique for imparting information onto a carrier wavelength, because it avoids the need and the expense of an external modulator for each wavelength in the system. However, at high bit transmission rates, direct modulation result in excessive linewidth broadening and wavelength instability which limits the wavelength spacing in WDM systems.
In early WDM systems, the wavelength spacing was also limited, in part, by the ability to effectively separate wavelengths from the WDM signal at the receiver. Most optical filters in early WDM systems employed a wide pass band filter, which effectively set the minimum spacing of the wavelengths in the WDM system.
The development of effective optical filters, namely in-fiber Bragg gratings, has provided an inexpensive and reliable means to separate closely spaced wavelengths. The use of in-fiber Bragg grating has further improved the viability of WDM systems by enabling direct detection of the individually separated wavelengths. For example, see U.S. Pat. No. 5,077,816 issued to Glomb et al.
A shortcoming of direct detection electrical regenerators schemes in WDM systems is that an optical filter and receiver combination must be provided for each wavelength. For example, see U.S. Pat. Nos. 5,063,559 and 5,504,609. Therefore, the cost of electrical regenerators in WDM systems increases in direct proportion to the number of wavelengths being used in the WDM point to point system.
Also, it is an industry desire to establish optical networks in which communications traffic can be flexibly accessed at points in the system other than terminals at which the optical signals are regenerated to compensate for optical noise. Therefore, the number of filters and receivers required in WDM networks and associated cost will have to be dramatically increased to provide the desired functionality.
However, the continued development of communications technology is dependent upon the continuing reductions in the cost of transmitting information. Thus, it is imperative that optical systems be developed that provide the increased flexibility required in next generation optical systems, but in a more cost-effective and upgradable manner that previously available in the industry.
The systems, apparatuses, and methods of the present invention address the above demand for improved optical systems. An optical transmission system of the present invention includes at least one transmitter configured to receive information from a plurality of optical receivers and transmit the information in a plurality of information carrying optical signal wavelengths. At least one optical receiver is configured to receive a plurality of information carrying optical signal wavelengths and a local oscillator signal. The optical receiver converts a plurality of optical signal wavelengths into a corresponding number of electrical signal frequencies.
In a number of embodiments, the optical receiver is used in combination with one or more second optical transmitters to provide a multiple signal wavelength regenerator. In various embodiments, the receiver/transmitter pair can be operated at the same or different optical signal wavelengths to provide optical signal regeneration or wavelength conversion between optical systems. In addition, tunable local oscillator and transmitters can be employed to provide an optical system having flexible wavelength allocation.
In various embodiments, one or more add/drop devices can be used in combination with the receivers to convert information carrying optical signal wavelengths into corresponding electrical signal frequencies. The add/drop devices can be configured to convert the optical signal wavelengths to electrical signal frequencies for use in other electrical or optical systems. The add/drop devices can also be used to interconnect sections of two optical systems, such as a ring to a trunk line.
In WDM systems of the present invention, multiple information carrying electrical signal frequencies can be separated and used to directly or externally modulate an optical source in the second optical system. In other embodiments, the multiple signal frequencies can upconverted using a single optical source/transmitter, thereby decreasing the number of transmitters, as well as receivers, used in the system.
Accordingly, the present invention addresses the aforementioned problems and provides apparatuses and methods to increase the efficiency and capacity of optical communication systems without commensurate increases in the cost of optical components. These advantages and others will become apparent from the following detailed description.